The present invention relates generally to a heating, ventilating, and air conditioning system. More particularly, the present invention relates to a method and system for controlling an economizer in a heating, ventilating, and air conditioning system.
Heating, ventilating, and air conditioning (HVAC) systems are used in both warm and cold climates to control the environment, including the temperature, within a building or other enclosure. HVAC systems typically include a heating unit for warming cold air and a cooling unit for cooling warm air. A fan pushes or pulls air over the heating or cooling unit and through a supply duct to the enclosure to condition the air within the enclosure. Air is circulated back to the heating or cooling units from the enclosure through a return duct. The HVAC system may also include an outdoor air damper, or economizer, that can be modulated to allow varying amounts of outdoor to mix with the air in the return duct to provide fresh air to the enclosure.
HVAC systems can be used to condition the air in a building of different sizes. Large buildings are often divided into a series of zones that are conditioned by the same HVAC unit. Each zone may include a thermostat or similar device, to sense and help regulate the condition of the air within the particular zone. Such a thermostat allows a user to select a desired temperature or temperature range for each of the individual zones.
In many cases, a single HVAC unit conditions the air within a number of different indoor spaces or zones. The air conditioned by the HVAC unit is fed into a main supply duct that subdivides into a network of smaller supply ducts that supply air to each individual space or zone. In some variable air volume ("VAV") systems having multiple zones, an air damper is placed in some or each of the smaller supply ducts leading into each individual zone. When the thermostat and/or controller for a zone determines that the temperature of the air in that zone is within the selected temperature range, the control for the system modulates the air damper to reduce or stop the amount of conditioned air entering the respective zone. Similarly, when the thermostat and/or controller for a zone determines that the temperature of that zone is outside of the selected temperature range, the damper is modulated to increase the flow of air into the zone.
When the temperature of a number of the individual zones is brought within the range selected for the particular zone and the dampers leading to many or each of the zones are closed or reduced, the fan continues to push air into the supply duct causing the pressure in the main supply duct to increase. Often, a bypass duct connects the supply duct with the return duct to allow air to circulate and relieve this pressure. The bypass duct usually includes a damper to control the amount of air circulating through the bypass duct. However, when air circulates from the HVAC unit through bypass duct and back to the HVAC unit, the returned air can become very warm or very cold, depending on the current operating state of the HVAC unit. Exposing the components of the HVAC unit to such very warm or very cold air can damage the unit, resulting in equipment failures and increased warranty costs for the components.
In light of the foregoing there is a need for a method and system for moderating the temperature of air circulating in a zoned HVAC system when the requirements of a number of the individual zones are satisfied and the return air becomes too hot or cold.